1. Field of the Invention
The present invention relates to an imaging lens for forming an optical image on a solid state image sensing device which is suitable for use for an onboard camera, a monitor camera, a mobile terminal camera and the like and a camera system which utilizes the imaging lens.
2. Description of the Related Art
Onboard cameras are known which pick up images of surroundings of a motor vehicle on which the cameras are installed to show the driver conditions surrounding the vehicle via images on a monitor screen (for example, refer to JP-A-2006-168683). These onboard cameras include, for example, an outside camera which is provided on an outside of the vehicle for assisting the driver to drive the vehicle by picking up images of surroundings which lie within a blind range to the driver and an inside camera which is designed to pick up images of conditions resulting when the vehicle is involved in a traffic accident in the same way as viewed by the driver. In recent years, in association with an increase in performance and a decrease in price of solid state image sensing devices, it is becoming a general trend to equip onboard cameras even on ordinary vehicles.
In the related art, imaging lenses like those described in, for example, Japanese Patent No. 3206930, JP-A-10-170821, JP-A-2001-337268 and JP-A-2005-321742 are known as small wide-angle lenses for use for onboard cameras, monitor cameras, mobile terminal cameras and the like. Imaging lenses described in Japanese Patent No. 3206930 and JP-A-10-170821 are such as to include, in order from an object side, a planoconcave lens and two positive lenses so as to allow a good telecentric characteristic to be compatible with compactness. In addition, imaging lenses described in JP-A-2001-337268 and JP-A-2005-321742 are such as to be made up of three lenses, of which a first lens lying nearest to objects is made up of an aspherical plastic lens.
With the imaging lens described in Japanese Patent No. 3206930, there is provided a drawback that the production costs are increased due to the use of a material having a high refractive index. With the imaging lens described in JP-A-10-170821, there is provided a drawback that the f number is f/3.4, which is dark. The imaging lenses described in JP-A-2001-337268 and JP-A-2005-321742 are both a wide-angle lens made up of three lenses, and the plastic lens is used as the first lens which lies nearest to objects. Therefore, there is provided a drawback when used as an outside onboard camera that the plastic lens provides poor weathering resistance and hence, the performance thereof is easy to be deteriorated.